I tried several times to use an image as layer mask, but it ain't working, even if I converting it to grayscale, the application doesn't want to paste it to the mask. Is it possible somehow?
What you need to do is to alt (option on mac) click on a layer mask thumbnail. You will then see the contents of the layer mask. You can then paste the contents of your clipboard, and a grayscale version will be pasted in the layer mask. Finally click on the artwork layer thumbnail for the artwork to display and the transparent masked items will show as a checkerboard pattern.